In recent years, attention has focused on a virtual network using software, and Software Defined Networking (SDN), which is a technique used to form a virtual network. Attention has also focused on a protocol such as OpenFlow or the like in order to form a virtual network. With SDN, a control device called a controller manages devices included in a network.
FIG. 1 illustrates an example of a control process in a network. The network illustrated in FIG. 1 includes one controller (OpenFlow controller) 5, and four switches (OpenFlow Switches) SW1 to SW4 managed by the controller 5. Moreover, a communication device 2a is connected to the switch SW1, while a communication device 2b is connected to the switch SW4. It is assumed that the OpenFlow protocol is used in the network illustrated in FIG. 1. Hereinafter, it is assumed that a process for communication between the communication device 2a and the communication device 2b is set in none of the switches SW1 to SW4 although the communication device 2a has transmitted a packet P1 to the communication device 2b. 
As indicated by an arrow A1, the packet P1 transmitted from the communication device 2a reaches the switch SW1. The switch SW1 makes an inquiry about a process of the packet P1 to the controller 5 by transmitting a control message including the packet P1 to the controller 5 (arrow A2). The controller 5 decides a transfer path of the packet P1 by using a topology of the network, and notifies the switches SW1 to SW3 that transfer the packet P1 of an output port of the packet addressed from the communication device 2a to the communication device 2b (arrow A3). Since the switch SW4 is connected to the communication device 2b, which is a destination of the packet P1, the controller 5 also transfers the packet P1 to be output to the switch SW4 when the controller 5 notifies the switch SW4 of the output port (arrow A4). The switches SW1 to SW4 store the information notified from the controller 5. Additionally, the switch SW4 transmits the packet P1 to the communication device 2b by outputting the packet P1 from the output port designated from the controller 5 (arrow A5). Next, when the communication device 2a has transmitted a packet P2 addressed to the communication device 2b, the switches SW1 to SW4 transfer the packet P2 by using the information notified in the arrows A3 and A4. Accordingly, the packet P2 is transferred to the communication device 2b via the switches SW1 to SW4 (arrow A6).
As described above, the switches managed by the controller 5 process a packet by using information obtained from the controller 5, and makes an inquiry to the controller 5 about a process for a packet for which a process target is not notified. Meanwhile, the controller 5 notifies a switch, which processes a packet that is inquired about, of content of the process for the packet.
As a technique related to management of a network, a technique such as LLDP (Link Layer Discovery Protocol) or the like is sometimes used to identify a connection relationship between devices included in the network.
The following documents are known as related techniques.    [Non-patent Document 1] “OpenFlow Switch Specification Version 1.3.2”, [online], Apr. 25, 2013, Open Networking Foundation <https://www.opennetworking.org/images/stories/downloads/sdn-resources/onf-specifications/openflow/openflow-spec-v1.3.2.pdf>    [Non-patent Document 2] IEEE Std 802.1AB, IEEE Standard for Local and Metropolitan Area Networks: Station and Media Access Control Connectivity Discovery, [online], Sep. 17, 2009, IEEE Computer Society <URL: http://standards.ieee.org/getieee802/download/802.1AB-2009.pdf>
The controller (control device) notifies all the switches that process a packet, for which a processing method is inquired about, of information for identifying the processing method. However, when a considerable amount of time is used for communication between the controller and some of the switches due to congestion or the like within the network, some of the switches receive the packet to be processed earlier than information from the controller in some cases. In this case, the switches make an inquiry to the controller about the process of the received packet. Therefore, the controller again executes processes such as calculation of a transfer path, and settings of a switch for the same packet, so that a processing load on the controller becomes heavy. Also, when a protocol other than OpenFlow described in BACKGROUND is used, a similar problem can occur in a network where a control plane and a data plane are separate.